Winning Strategy toward Acentric Crystals: Transverse Dipole Moment Molecules

Abstract

Imines obtained by condensation of 4-hydroxybenzohydrazide with carbonyl compounds are a rare example of non-chiral compounds showing a remarkable tendency to crystallize in acentric and polar space groups, as we underlined in a previous study of imines obtained by condensation with aliphatic ketones. Here, we present an extensive study of imines obtained by condensation with aromatic aldehydes and demonstrate that over a representative set of 60 crystal structures, the overall occurrence of acentric phases in this class of compounds is 47%. This figure is remarkably higher than the overall 22% frequency of acentric structures in the Cambridge Structural Database and pinpoints a successful strategy toward the synthesis of acentric crystals. In the crystal structures of the acentric imines, complex chains of H-bonded molecules are generally present, with formation of 2D H-bonded networks, similar to other classes of compounds described in the literature with a propensity to form acentric crystals. Another reason for the high success in achieving acentric and polar structures in this class of compounds can be related to the presence of strong transverse bond dipole moments. This feature allows a polar packing of the long molecular axes while keeping the bond dipole moments antiparallel.

Ultradynamic Isoreticularly Expanded Porous Organic Crystals

“…Consequently, this packing facilitates the formation of a "T/M/M/T" sequence, thereby effectively minimizing the packing dipole moments. 46 Nonetheless, the successful isoreticular co-crystallization over three generations of tricarboxylic acid building blocks showcased the benefit of the employment of heteromeric complementary hydrogen bonding moieties.…”

“…These conformational differences between TAB and TPB derivatives influence the overall crystal packing, and the M2/M3-TAB co-crystals exhibit a closer stacking between the M2 and M3 molecules. Consequently, this packing facilitates the formation of a “T/M/M/T” sequence, thereby effectively minimizing the packing dipole moments . Nonetheless, the successful isoreticular co-crystallization over three generations of tricarboxylic acid building blocks showcased the benefit of the employment of heteromeric complementary hydrogen bonding moieties.…”

Elucidating the Polymorphism of Xanthone: A Crystallization and Characterization Study

“…Different crystal structures of the same compound can exhibit very different physical and mechanical products. Hence, establishing the solid forms landscape of a compound (including polymorphs, solvates, hydrates, co-crystals) and the transformation pathways between each of these forms is an increasingly important part of development of a new product. − Furthermore, tailoring the crystal structure of a compound in order to achieve targeted physicochemical properties is a key challenge for crystal engineers. − …”

“… 10 − 13 Furthermore, tailoring the crystal structure of a compound in order to achieve targeted physicochemical properties is a key challenge for crystal engineers. 14 − 17 …”

Frustration of H-Bonding and Frustrated Packings in a Hexamorphic Crystal System with Reversible Crystal–Crystal Transitions

“…One potential factor contributing to polymorphism is the phenolic donor, which, in the cases we have studied so far, can take two different conformations in the plane of the phenyl: in the same side of the carbonyl acceptor (H a ) or in the opposite side (H b ). ,− Another factor is that the two acceptors are only three bonds apart, so, depending on small structural variations in the molecule, they can act as acceptors from the same (bifucated) donor. Amido NH is always a donor only to carbonyl O. Phenolic OH is a donor to imino N, but in some cases, it is a bifurcated donor both to imino N and carbonyl O, the latter being a bifurcated acceptor.…”